My B.F.F Daniel and I were actually planning on paying a visit to the abandoned coalmine of Hasard in the village of Cheratte, Belgium; but for some reason it didn't really happen. When we arrived in Cheratte, however, we noticed an abandoned and decaying office building just down the road. We decided to check that out instead of the Hassard complex.
Getting in was easy, the factory next door had its gate open and we could just walk into the office building via the back entrance. While we were inside, we heard a car/truck driving really close. When we were ready to leave, we discovered that the gate was closed. Fortunately for us, they (people working at the factory, I presume) placed a huge concrete block in front of the gate. This was without doubt to prevent people from getting in, but it also made the climb over the fence a lot easier 
Thanks dude!